1. Field of the Invention
The present invention is directed to an anti-interference filter for radio-shielding a feeder to a gradient coil in a nuclear magnetic resonance imaging apparatus.
2. Description of the Prior Art
In a typical magnetic resonance imaging apparatus for producing tomograms, radio-frequency nuclear magnetic resonance signals are caused to be emitted from a patient, and are received by an antenna and are evaluated for generating a tomogram of the subject. The nuclear magnetic resonance signals are of extremely low power, so that particular attention must be paid to conducting an undisturbed processing of those signals. In particular, introducing interference to the reception antenna must be avoided. The antennas or resonators for exciting and picking-up the nuclear resonance signals, and the coils for generating the required magnetic fields, are therefore in conventional systems arranged in a radio-frequency-tight compartment, into which unwanted signals cannot penetrate. This also assures that the radio-frequency fields generated by the antennas cannot proceed to the exterior of the enclosure as an unwanted signal.
The coils for generating the gradient magnetic fields, referred to herein as gradient coils, are supplied via a gradient power supply disposed outside the radio-frequency enclosure, so that it is possible for interference signals to proceed into the interior of the radio-frequency enclosure via the feeders to the gradient coils. In order to suppress this interference, it is known to use anti-interference filters connected between the feeders and the gradient coils.
Certain boundary conditions must be observed in the design of such anti-interference filters for the feeders to the gradient coils. A high direct current, given low d.c. attenuation and high radio-frequency blocking attenuation, must be conducted into the radio-frequency enclosure via the anti-interference filter. Due to saturation as a result of the high static magnetic field in the magnetic resonance imaging apparatus, and the high currents required for excitation of the gradient coils, inductances in the anti-interference filter cannot be constructed with ferrite materials. The gradient fields are activated and deactivated in a defined sequence during a transmission/reception cycle, and the anti-interference filter for the gradient power supply can therefore only represent a maximum capacitative load. The values of the capacitance in the anti-interference filter are dependent on the nominal operating data for the gradient power supply, and should not exceed a maximum value.
Known anti-interference filters are constructed as low-pass filters, which prevent all frequencies higher than a defined, lower limit frequency from entering the radio-frequency enclosure. The lower limit frequency is substantially set by the leading or trailing edge of the gradient pulse in the activation or deactivation of the gradient coils. Low-pass filters are particularly employed in magnetic resonance imaging systems in order to evaluate the nuclear magnetic resonance of the nuclei of different substances. Such filters are insensitive to tolerances, and therefore need not be manufactured or tuned with great precision. The coils employed in the low-pass filters, however, occupy a large amount of space, because high inductance values must be used, given a low d.c. resistance. Moreover, in order to achieve a blocking attenuation, low-pass filters of a higher order must be used. The coils of such higher order filters must be individually shielded, because their inductances would otherwise be reduced due to mutual couplings. For this reason, only low-pass filters of at most the fifth order have heretofore been used.